1. Field of the Invention
The present invention relates to an image processing circuit and, more particularly, to an image processing circuit for use in a liquid crystal display.
2. Description of the Background Art
In recent years, liquid crystal displays have been utilized in various fields and have been utilized in televisions as well as in PC monitors. However, liquid crystal displays have low response speeds, thereby including the problem of degradation of display quality due to afterimages in cases where moving images are mainly displayed thereon as in TV applications. Therefore, overdrive processing methods have been applied to liquid crystal displays, in order to increase their response speeds. Overdrive processing is a processing method for, in cases where image data is moving images, setting a voltage applied to a liquid crystal to be higher than usual if a direction of data change from a previous frame to a current frame is positive, but setting the voltage to be lower than usual if the direction of data change from the previous frame to the current frame is negative. This method can improve display quality of moving images.
As overdrive processing commonly applied to liquid crystal displays, there is a method for calculating an amount of overdrive using a lookup table (hereinafter, simply referred to as “LUT”). However, this method requires an LUT provided in accordance with the number of gray-scale of image data, which has induced the problem of increases of data due to great numbers of gray-scale. For example, in cases of image data of 6 bits (64 gray-scale), there is need for providing an LUT storing 4096 pieces of data for the combinations of 64 gray-scale of a current frame and 64 gray-scale of a previous frame.
Consequently, image data has been quantized with predetermined threshold values and an LUT has been applied to such quantized data to reduce the amount of data in the LUT. More specifically, image data of 64 gray-scale is divided into 8 sections and converted into 3-bit quantized data, by quantizing it with seven threshold values.
Further, it is known that the response speeds of liquid crystal displays depend on the temperature. Namely, the response speed of a liquid crystal display increases with increasing temperature, while the response speed of the liquid crystal display decreases with decreasing temperature. Accordingly, there has been caused the problem that proper overdrive processing can not be performed when an LUT which has been optimally set in a room temperature condition is used at higher temperatures (for example, +60° C.) and lower temperatures (for example, −20° C.).
Consequently, conventional overdrive processing has employed, as methods for correction with respect to temperature changes, a method of correcting output values from LUTs through calculations and a method of providing LUTs in association with respective temperatures as described in Japanese Patent Application Laid-Open No. 2004-133159.
However, the method of correcting output values from LUTs through calculations corrects all gray-scale with the same ratio. Consequently, the method has had the problem of insufficient correction of the temperature characteristic of the response speed of a liquid crystal display which varies depending on the gray-scale.
Further, the method of providing LUTs for respective temperatures has had the problem of increases of the size of an SRAM or the like for storing the LUTs and, therefore, increases of the circuit scale.